Stretched membranes are a class of filtering membranes. Stretched membranes are typically made by first forming a non-porous hollow fibre of an appropriate membrane material. The membrane material is typically a thermoplastic such as polypropylene (PP), polyethylene (PE) or, less frequently, PVDF. The membrane material can be formed into a nonporous hollow fibre by various methods such as extruding or melt-spinning. The non-porous hollow fibre (often called a precursor) is then treated by a variety of processes which typically include annealing steps and stretching steps (typically at specified temperatures, rates of elongation, and total elongation) to produce pores within a desired size range in the fibres. The processes generally share a common strategy of providing a non-uniform composition in the fibre with areas having a crystalline structure dispersed throughout the fibre. When the fibre is stretched, the membrane material between the crystalline structures tears into a series of microfibrils alternating with elongated pores oriented along the length of the fibres.
A large number of specific processes have been described in the art. While following the generally strategy outlined above, these processes may differ, among other things, in the number of annealing or stretching steps, the order of the annealing or stretching steps and the temperature, changes in temperature, total elongation or rate of elongation specified for any particular stretching step.
A typical plant for making stretched membranes allows for generally continuous production. For example, in one area of the plant, precursor may be formed by extruding a continuous fibre that passes over a series of rollers designed to stretch the fibres to a final dimension (but without producing pores) and allow the fibre to cool before it is wound around a take up spool. Once full, the take up spools are then mounted into the head of a stretching apparatus. The stretching apparatus may consist of a series of chambers each having a plurality of rollers of differing diameters and turning at different speeds. The fibre is threaded through the various rollers of the chambers in series to a take up roll at the tail end of the stretching apparatus. The different speeds and diameters of the rollers in each chamber causes the fibre to be stretched (or not stretched for annealing) at selected speeds and to selected elongations as it passes through each chamber. The temperature of each chamber is individually set as desired for stretching or annealing. The total length of the path (around the various rollers) in each chamber and the average speed of the fibre through each chamber can be chosen to achieve a desired length of time that the fibre is exposed to the temperature prevalent in each chamber. As long as the fibre does not break, a continuous fibre of stretched filtering membrane winds continuously onto the take up roll at the tail end of the plant. Once a roll of fibre is produced, membrane modules can be made by various techniques.